Generally, oxygen is delivered from a pressure-regulated source at a continuous flow rate prescribed by a physician. These pressure-regulated sources include large stationary oxygen cylinders, small portable oxygen cylinders, stationary liquid oxygen reservoirs, small portable liquid oxygen containers, oxygen concentrators, and oxygen piped through hospital walls.
Since the respiration cycle is approximately one-third inhalation and two-thirds exhalation, and oxygen provided during exhalation does not benefit the patient, a substantial amount of oxygen is wasted during exhalation.
Previously, flow control valve devices administered oxygen via a respirator, or scuba type equipment required a face mask in order to detect inspiration. Devices with adequate sensitivity to detect inhalation through a nasal cannula could not distinguish negative pressures produced by inhalation from those that occurred as a result of the valve opening and closing. Some devices avoided this problem by delivering a fixed volume of oxygen very early in the inspiration cycle, but the amount of oxygen required varies from patient to patient. It is generally accepted that the best treatment is the one that is the most like normal breathing and at the same time maintains as close to normal oxygen saturation in the patient's blood as possible.